This invention relates to voltage surge arresters and particularly to those comprising a series stack, or a plurality of series stacks in parallel, of metal oxide arrester blocks without spark gaps.
There has been concern in the past with monitoring the field condition of lightning arresters. It is desirable to know if the arrester is in good condition and doing its protective function. Kennon U.S. Pat. No. 3,443,223, May 6, 1969, is directed to a lightning arrester leakage current and duty monitoring system that includes an instrument package in series with the arrester for the detection and mesurement of leakage current and also for counting the number of times the arrester has discharged. Additional or alternative monitoring capability is desirable, particularly in recently introduced arresters of the type comprising a series stack of metal oxide varistors without spark gaps. The varistors comprise zinc oxide and other metal oxide materials and have a favorable non-linearity characteristic as compared to previously used silicon carbide. As the varistor blocks themselves, rather than any associated spark gaps, are the sole protective elements in the arrester, it is of considerable interest to be able to monitor their condition in the field. This desirability is more pronounced because metal oxide blocks are susceptible to a form of deterioration characterized by a gradual increase in leakage current until thermal runaway eventually occurs which may result in destruction of the blocks.
It has been recognized that metal oxide blocks have an essentially capacitive type characteristic when operated below their turn-on level of voltage, above which they produce a low resistance path. It is therefore known, and utilized in the laboratory, that one may observe the voltage across a zinc oxide block and the phase relation of that voltage with the current to determine if the block is maintaining its desirable substantially capacitive characteristic or is highly resistive. As deterioration occurs, the resistive component of current will increase, gradually exceeding the capacitive current and finally generating more watts of heat resulting in thermal runaway. The use of this lab technique in the field with metal oxide arresters as heretofore constructed has been cumbersome and expensive. It has required a magnetic voltage transformer or a capacitive voltage transformer, normally costing thousands of dollars, to get the necessary voltage indication.
Briefly, in accordance with the present invention, a much simpler yet effective means is provided for giving the voltage indication that can be compared with a coincident current indication to permit a determination of the arrester block condition. This is by utilizing the stack of metal oxide arrester blocks itself as a capacitive voltage divider. Basically, all that is required is an additional voltage tap between the last or bottommost block of the series stack and the adjacent block so that voltage readings may be taken across the last block as desired. Additionally, it is merely necessary that a resistive shunt be connected in series with the stack of blocks, such as between the last block and the ground terminal, so as to permit current detection as has sometimes been done in prior arresters.
A surge arrester so equipped may be utilized in various ways. A principal way is to directly compare tracings of voltage and current over a time period such as on an oscilloscope or recorder in order to determine the phase relation between the voltage and current and to ascertain the condition of the arrester blocks.
A further manner of using an arrester constructed as described above is to have a relay connected between the voltage tap and the ground terminal that is responsive to a predetermined voltage magnitude to conduct and indicate at least normal voltage between the line and ground terminals. In this method of use, upon the detection of a voltage which is less than the threshold normal voltage there is a voltage drop which would be insufficient to close the relay or to keep the relay closed. This would occur upon the occurrence of a fault.
A further use of arresters constructed in accordance with this invention is to measure the magnitude of the tap voltage and to provide by instrumentation or manual calculation a multiplication of that tap voltage by the capacitance ratio (C.sub.1 +C.sub.2)/C.sub.1 where C.sub.1 is the capacitance of the stack of arrester blocks above C.sub.2 and C.sub.2 is the capacitance of the arrester block across which the tap voltage is taken. This permits the user of the equipment to determine the line voltage magnitude without other equipment. Furthermore, the measurement of current and voltage in the manner described permits detecting and recording the current magnitude upon a current surge when the voltage is clamped to a value indicating occurrence of a fault and a discharge of the arrester. It is useful to the arrester user to know the magnitude of the current surge for the general purpose of monitoring system performance.
A still further method of use is that of employing the measured current and voltage to provide, by the integration of the product thereof over a period of time, the magnitude of energy absorbed by the arrester in that period of time. This is a useful indicator of the energy in a surge, as well as usage of the arrester and can be helpful in determining its available service life.